The present invention relates to rotary fluid-driven motors and, in particular, it concerns rotary water-driven or air-driven motors which employ sealing elements.
Rotary hydraulic motors are motors which are driven by static fluid pressure. In other words, they are designed geometrically such that the balance of surfaces acted upon by the inlet liquid pressure is always eccentric to the axis of rotation. The product of the balance of surfaces and the liquid pressure together with the eccentricity (the perpendicular distance of the balance of surfaces from the axis of rotation) generates a net moment in the direction of rotation.
Known types of hydraulic motor operating according to these principles include various types of vane motors and gear motors. Motors of these types tend to suffer from internal leakage from the high-pressure inlet region to the low-pressure outlet region. Leaks of this kind do not perform “work”, i.e., they do not contribute to positive displacement of the parts of the motor, and they therefore reduce power efficiency of the motor. Accordingly, such leaks need to be minimized by internal sealing mechanisms within the motor.
The conventional approach to achieving effective sealing within rotary hydraulic motors is by use of high precision components with very small clearances between the moving parts and the motor casing. Since the flow rate of the leaks is a function of the size of the clearances between parts, the leakage rates can be reduced by employing very small clearances. Nevertheless, this approach tends to inherently allow some degree of leakage. Typically, the volumetric efficiency curve for motors of this type is poor at low power and rises asymptotically towards a maximum value at higher flow rates (see FIG. 1). Since most oil-based hydraulic systems are high-power systems employing operating pressures of at least tens, if not hundreds, of atmospheres, the energy losses from leakage are typically not particularly problematic.
In the field of domestic and garden automation, there is a trend towards devices which are powered by water-driven or air-driven motors actuated by connection to a domestic water supply or a supply of compressed air. In order for such devices to be lightweight, cost effective and to avoid corrosion, it would be advantageous to produce water-driven motors primarily or exclusively from injection molded plastic components. This however presents problems due to the relatively wide manufacturing tolerances which must be allowed for due to the current limitations of plastic injection molding technology. This problem is further exacerbated by the low pressure fluid supply, typically in the range of 2-8 atmospheres in the case of a domestic water supply. This combination of low driving pressure and wide manufacturing tolerances renders the implementation of static-fluid-pressure-driven motors for low-cost domestic applications particularly problematic.
As an alternative to static-fluid-pressure driven motors, many existing water-driven devices employ turbine-type motors where a rotor is driven by kinetic energy transferred from a flow of water impinging upon the rotor blades. Such a device is necessarily not sealed, and therefore does not require high precision manufacturing techniques. Turbine-type devices, however, offer very low efficiency and are particularly problematic at low flow rates.
There is therefore a need for rotary hydraulic motors produced primarily from injection-molded plastic components which would offer effective sealing under domestic water-actuated or air-pressure-actuated operating conditions.